The present invention relates to a ring resonator gyroscope.
A known ring resonator gyroscope will now be described with reference to FIGS. 1 and 4 of the accompanying drawings in which:
Referring to FIG. 1, a ring resonator gyroscope is indicated generally at 10 and operates by inserting laser light into a resonator so as to form clockwise (CW) and counterclockwise (CCW) beams. The ring resonator gyroscope 10 comprises a fibre resonator 12 formed by connecting a length of optical fibre 14 back onto itself in a coupler 16 which has a high coupling ratio.
A laser 18 produces a laser beam of narrow linewidth which is split at a beamsplitter 20 to allow insertion of light in both directions in the fibre resonator 12 and also to allow separate monitoring of the light in each direction.
Acousto-optic deflectors 22 and 24 are placed in each path to allow the relative frequency of the CCW and CW beams respectively to be adjusted. The CW beam is deflected by a mirror 25 to the deflector 24. Microscope objectives 26 and 27 focus laser light onto the ends of the fibre 14.
Portions of the CW and CCW beams exit the fibre resonator 12 via the coupler 16. Beamsplitters 28 and 29 reflect the exiting CW and CCW beams into photodetectors 30 and 32 respectively.
The output intensity is derived from the coherent addition of all of the combined delayed waves of varying amplitude and phase and depends on the coupling ratio of the coupler 16, the wavelength of the laser source and the delay time of the fibre resonator 12.
FIG. 3 shows the resonance characteristics of a fibre resonator. Changing the phase path of the fibre resonator by changing the length of the fibre resonator or the source wavelength will vary the resonance characteristic because of the consequent change in the relative phases of the combining waves. When the phase path is an integral number of wavelengths the resonance characteristic exhibits a null. The intensity inside the loop has an inverse function to that of the output. At resonance, zero output intensity, the wave continually recirculates within the loop, and is lost by scattering from the loop.
Resonant peaks occur at frequencies spaced by the `free spectral range` (FSR) of the fibre resonator where: EQU FSR=c/nL
where c is the velocity of light
n is the fibre refractive index PA0 L is the length of the fibre resonator. PA0 two low noise amplifiers 42 and 44 to which the photodetectors 30 and 32 respectively are connected; PA0 a differential amplifier 46 receiving the output of the amplifiers 42 and 44; PA0 an oscillator 48 operating at frequency Wc connected to two lock-in amplifiers 50 and 52 the inputs of which are connected to the outputs of the low noise amplifier 42 and the differential amplifier 46 respectively; PA0 two integrators 54 and 56 having inputs connected to the lock-in amplifiers 50 and 52 respectively; PA0 a high-voltage amplifier 58 having inputs connected to the oscillator 48 and integrator 54 and an output connected to the PZT 34; PA0 a voltage-to-frequency convertor 60 having an input connected to the integrator 56 and an output connected to the acousto-optic deflector 22.
The quality of the cavity is defined by the `finesse` (F) which is the ratio of the FSR to the resonance line width (.DELTA.f): ##EQU1##
If a typical fibre resonator is considered, with a loop length of 10 meters, the spacing between modes is about 20 MHz, and if the finesse of the resonator is 100 the halfwidth of the resonance is 200 KHz.
In FIG. 1, the light impinging on the photodetector 30 is used to control the path length of the fibre resonator 12 to maintain resonance. This is achieved by causing an electrical control signal to be supplied to a cylindrical piezoelectric transducer (PZT) 34 around which the fibre 14 is wound and which is operable to stretch the fibre 14.
Referring to FIG. 2, a frequency and path length control system is indicated generally at 40 and comprises:
In FIG. 2, the difference in intensity of the CW and CCW beams sensed by the photodetectors 30 and 32 at frequency Wc is used as the error signal to drive the frequency of the acoustic optic deflector 24 to the minimum of the CW resonance.
The path length is altered by applying a sinusoidal modulation on the PZT 34 at a frequency Wc (typically 10 kHz) via the high voltage amplifier 58. If the path length is not held accurately, there will be a signal on the photodetector 30 at frequency Wc, the sign of which determines which side of the resonance is the fibre length. FIG. 4 shows the variation of the light intensity at frequencies Wc and 2Wc as a function of detuning from line centre. The former goes through zero at line centre, and so can be used as an error signal to drive the path length to line centre. The arrangement shown in FIG. 2 achieves this by a synchronous detection scheme using the lock-in amplifier 50 with the reference signal at frequency Wc. If there is offset in the path length on the pathlength servo (so that the servo is not on the exact minimum of the CW resonance) then this should also appear on the CCW beam, as the path length affects both directions equally so that the signal driving the frequency servo (ie. driving acousto-optic deflector 24) is only responsive to the non-reciprocal signals.
Acousto-optic deflector 22 is held at a fixed frequency for the CCW beam. The light in the CW beam goes through acousto-optic deflector 24 which gives a variable frequency offset according to the output from the voltage-to-frequency convertor 60.
A disadvantage of the arrangement shown in FIG. 2 is that it only gives a small amount of common-mode rejection which depends upon the relative gains of the two low noise amplifiers 42 and 44. Also there may be slight differences in the intensities of light falling onto the two photodetectors 30 and 32 which will again reduce the effectiveness of this scheme to reduce common mode signal errors.
It is an object of the present invention to provide an improved frequency control system for a ring resonator gyroscope.